What are the different alternative techniques for determining location using cell phones? Explain the role of beacon signals.

Question

What will be an ideal response?

Answer 1

Yes, the techniques used by satellites, can also be used by the cell phone BSs in determining the location of a MS. This implies that the MS needs to be observable by at least 3 adjacent BSs (preferably 4). Relative distance, angle, or phase of the signal from a MS to 3 BSs can be used to compute the obsolete location of the MS and beacon signals can help measuring these parameters. Designing a geo-location system could be much easier if the receiver knew roughly where it was and what signals it should be looking for. Hence, assisted GPS system has recently been proposed to handle the need to locate cell phone within a few tens of meters. Assisted GPS leverages the following facts:

(a) The nodes have a means to communicate with an outside server
(b) The position of node relative to the outside server is roughly known and
(c) It is possible and inexpensive to build high quality GPS receivers to the outside servers to assist in determining the location of the node.

The hard problem is finding the satellite’s signal in the background noise. Yet if the rough location of the node relative to the server is known, the server could calculate the signal that receiver should see. With this added information, the receiver’s search space is much smaller and the receiver can actually make intelligent guesses about where the signal is. In the cell phone system, the system roughly to switch between cells. The BS would know the visible satellites and their Doppler frequency shift, which could be fed to the receivers to make it easier from them to find the needed signals. Other issues include the
need of synchronization between BS clock and MS clock and elimination of multipath signals.